Amperometric sensors have been used to detect the presence of specific analytes, for example enzymes in liquids, for over 20 years. The basic principle is to effect a reaction between the analyte to be detected in a sample and the sensor surface. The subsequent charge produced is then converted into a sensor current that can be measured. The size of the current is generally related to the quantity of analyte present.
FIG. 1 shows a simplified biasing circuit used in a typical sensor. The circuit consists of three electrodes, a counter electrode C, a work electrode W and a reference electrode R. The counter electrode C and reference electrode R are connected to a work potential setting amplifier, Amp1, and an output buffer amplifier, Amp2, and a current sensing resistor, Rsens as shown in FIG. 1.
The work electrode W is coated with a reaction inducing coating that reacts with the chosen analyte. For example, a glucose sensor might have a glucose oxidase coating on the work electrode W. The reaction produces ions that when subjected to a potential difference give rise to current flow from the counter electrode C to the work electrode W. The current also flows through the current sensing resistor Rsens giving a voltage drop Vout across that resistor. A typical sensor current might be 10 nA, and for Rsens=1 MΩ, Vout is 10n×1M=10 mV referenced to ground. As already noted the size of the current flowing from the counter electrode C to the work electrode W, and hence the output voltage Vout across Rsens, depends on the concentration of the analyte.
The potential on the reference electrode is key to achieving optimum sensor performance. The reaction efficiency at the working electrode W depends on the work potential VRW. Different sensors operate best at different values of work potential VRW. For example, a glucose sensor operates optimally at VRW=0.6V compared to VRW=−0.6V for an oxygen sensor. The role of the work potential setting amplifier is to maintain the work potential VRW at the value for which reaction conditions are optimised. This is done by setting the positive terminal of the work potential setting amplifier to Vref=VRW+Vout and the negative terminal of the work potential setting amplifier to VRW. As mentioned earlier, Vout is the potential drop across the current sensing resistor Rsens due to the sensor current and typically has a value of 10 mV. If the maximum voltage that can be generated across the current sensing resistor by a glucose sensor is, for example, 100 mV, Vref would be set to Vref=0.6V+100 mV to ensure that the reaction conditions are optimised. However, because Vout varies with, for example, analyte concentration and time, the work potential VRW is subject to fluctuations. The fluctuations of the work potential VRW away from Vref are a problem with sensor circuit designs such as that of FIG. 1 as they are detrimental to an efficient reaction at the work electrode W and also impact on the consistency of the output signal.
In practical applications of the sensor, a user may want to measure, for example, both the glucose and oxygen levels using the same sensor system. As already noted, glucose and oxygen sensors operate at different work potentials VRW, 0.6V and −0.6V respectively. The sensor system should therefore be able to accommodate both work potentials. If a single circuit of the type shown in FIG. 1 is used, a headroom of more than 1.2V would be required of the circuit voltage supply. However, this would be too large for a single chip low power integrated circuit design running at low voltages of 1V and below.
A possible solution to the voltage limitation problem referred to above is to make a multiple sensor by designing two parallel circuits on a single chip. This is illustrated in FIG. 2. Implementing this solution would require re-referencing the Vref and Vout signals to ground. However, using two parallel circuits increases the used chip area and therefore the costs of producing the chip. In addition to this, the design in FIG. 2 would still not solve the problem caused by fluctuations in the work potential VRW.
A further problem which arises with the designs of both FIG. 1 and FIG. 2 is the presence of Shott noise which results from the current sensing resistor. This noise is proportional to the value of the resistance and will be significant relative to the output voltage.